1. Field of the Invention
The present invention relates in general to the selection of waveguide modes in optical fibers. In particular, the present invention relates to an apparatus for selection of waveguide modes for light transmitted in an optical fiber and its corresponding method of fabrication.
2. Technical Background
The distinctive anisotropic characteristics of liquid crystals makes it an interesting material for exploitation in a wide range of applications. Studies exploring the use of liquid crystal materials in the making of optical fibers for various light-transmitting characteristics have been conducted. Liquid-crystal-clad tapered fibers, directional couplers with liquid crystal sandwiched between two side-polished fibers, coaxial fiber couplers embedded in liquid crystal, and optical fibers with liquid crystal core, are among the many subjects explored.
The control of polarization, or transmission mode, of light transmitted along an optical fiber is of potential importance in applications such as interferometric fiber sensors, as well as coherent light communication systems. Conventional means of polarization control is to emit light transmitted in an optical fiber and then pass it through a series of conventional bulk polarizers. After that, the light beam is redirected back into the optical fiber. Obvious disadvantages of the prior art means are that there are excessive insertion loss and difficulties in light re-alignment, as well as poor mechanical stability. To overcome these disadvantages, fiber-type polarizers directly compatible with fiber-optic transmission system are developed.
FIGS. 1a and 1b, for example, in the drawings accompanying the present invention show respectively the side view and cross-sectional view of the structural configuration of a typical prior art fiber-optic polarizer. A portion of the optical fiber 1 in the region near to the core 2 thereof, a portion of the cladding 3, is removed and replaced by birefringent materials such as potassium pentaborate (KB.sub.5 O.sub.8.4H.sub.2 O) crystal 4, which provides for the leaky loss when light is transmitted through the fiber.
FIG. 2 in the accompanying drawings is a side view showing the structural configuration of another prior art fiber-optic polarizer. A portion of the cladding 12 of an optical fiber is removed to expose the core, which also has a portion of its surface removed. A thin metal film 14 is then formed on the exposed surface of the core 16 to produce a metal-clad fiber-optic polarizer. The metal film provides radiative loss of light transmitted through the fiber. Light transmission having pseudo TE.sub.0 and pseudo TM.sub.0 mode components, as indicated by reference character M in FIG. 2, is polarized by this polarizer, with its pseudo TE.sub.0 mode component leaking out and the pseudo TM.sub.0 mode component remaining transmitted through the optical fiber, as indicated by N in the drawing. This is because the pseudo TE.sub.0 mode component does not conform to the cutoff condition of the metal-clad fiber-optic polarizer, which forms a cutoff polarizer with high extinction ratio.
The prior art polarizers, illustrated in FIGS. 1 and 2 and described above, manipulate the control of polarization over the HE.sub.11 modes in the fiber-optic waveguide. The resulting polarized light therefore is unidirectionally distributed in space and no polarization of azimuthal direction can be achieved. In polarization-dependent fiber-optic systems, polarization of the transmitted light in the azimuthal direction is potentially important, because of the characteristics of spatially circular symmetry in optical polarization direction and intensity distribution. In general, transmission of azimuthally polarized light can be obtained by selectively extracting the TE.sub.01 mode of the light in the fiber-optic waveguide. However, because the propagation constants, as well as the cutoff conditions in the normalized frequency, of the TE.sub.01, TM.sub.01 and HE.sub.21 modes of light transmitted over the conventional multimode optical fibers are similar, it is difficult to separate these modes. Without any practical means for extracting out the TE.sub.01 mode in existing state-of-the-art techniques, light polarization in the azimuthal direction can not be achieved.
The current in-line polarizers, mentioned previously, attempt to maintain the polarization of processed light in the defined directions by producing differential transmission loss between the mutually orthogonal polarization modes, namely the pseudo TE.sub.0 and the pseudo TM.sub.0 modes of the transmitted light, to provide for the selection of one. For example, J. R. Feth et al. disclosed a thin-film metal-clad fiber-optic cutoff polarizer and its corresponding method of fabrication in an article entitled "Metal-clad fiber-optic cutoff polarizer", published in Optics Letters, Vol. 11, No. 6, June 1986, pp. 386-388. Feth et al. achieved an extinction ratio of 47 dB with insertion loss of 1 dB in their polarizer. In another article entitled "Single-mode-fiber evanescent polarizer/amplitude modulator using liquid crystals", published in Optics Letters, Vol. 11, No. 3, March 1986, pp. 180-182, K. Liu et al. disclosed an in-line polarizer utilizing the birefringent properties of a nematic liquid crystal placed in the evanescent field of a single-mode fiber. Amplitude modulation was also shown using an external electric field to reorient the liquid crystal molecules. A polarization-extinction ratio of 45 db with an insertion loss of 1 db was achieved by Liu et al. T. Hosaka et al. in still another article entitled "Fabrication of single-mode fiber-type polarizer", published in Optics Letters, Vol. 8, No. 2, February 1983, pp. 124-126, proposed a polarizer formed from a single-mode fiber composed of a concentric core and a silica cladding with a B.sub.2 O.sub.3 -doped silica portion. The cladding was asymmetrically etched off with 49% HF by taking advantage of differential etching rates of pure silica and B.sub.2 O.sub.3 -doped silica. An Al film was subsequently deposited on the area where the cladding had been etched away. In their polarizer, Hosaka et al. showed a maximum polarizing extinction ratio of 37 dB at wavelength=1.29 .mu.m for a 4-cm polarizer. In still another study, W. Eickhoff disclosed a polarizer formed by grinding off the cladding on one side of a single-mode fiber and depositing metal onto the polished surface in the article "In-line fibre-optic polariser", published in Electronics Letters, Vol. 16, No. 20, September 1980, pp. 762-764, achieving an extinction ratio of 14 dB between orthogonal polarizations.
On the other hand, C. K. Asawa et al. showed techniques for coupling light of different modes or modal groups into or out of a multimode optical fiber, in U.S. Pat. No. 4,942,623, issued Jul. 17, 1990 entitled "Device and method for modal separation and combination in an optical fiber intrusion detection system". Moreover, in U.S. Pat. No. 3,891,302, issued Jun. 24, 1975 entitled "Method of filtering modes in optical waveguides", F. W. Dabby et al. showed planar waveguide filters and fiber-optic filters for filtering modes in an optical waveguide by selecting and providing a periodic variation such as surface corrugation or refractive index variation in a filter region in the waveguide. However, among all these disclosures, none reports the use of liquid crystals with radial molecular orientation and cylindrical symmetry in fiber cladding, to achieve mode selection or polarization control.